The present invention relates to a method for growing a nitride compound semiconductor. More particularly, it relates to a new and improved method for growing a GaN or other nitride III-V compound semiconductor.
GaN, AlGaN, GaInN and other nitride III-V compound semiconductors have band gaps ranging from 1.8 eV to 6.2 eV, and they theoretically may be used to provide light emitting devices capable of emitting infrared to ultraviolet light and for this reason, these metal nitride compound semiconductors are being studied by engineers.
To fabricate light emitting diodes (LED) or semiconductor lasers using nitride III-V compound semiconductors, it is necessary to stack layers of AlGaN, GaInN, GaN, etc. and to sandwich a light emitting layer (active layer) between an n-type cladding layer and a p-type cladding layer.
To grow a p-type GaN layer, for example, by metal organic chemical vapor deposition or other vapor phase growth in a conventional technique, trimethylgallium (TMG, Ga(CH.sub.3).sub.3) as Ga source, ammonia (NH.sub.3) as N source, and cyclopentadienyl magnesium (CP.sub.2 Mg) as a p-type dopant, for example, are supplied onto a heated substrate, such as sapphire substrate, SiC substrate or GaAs substrate, in hydrogen (H.sub.2) carrier gas or mixed gas containing H.sub.2 and nitrogen (N.sub.2), to grow a Mg-doped GaN layer by heat decomposition reaction. Since the Mg-doped GaN layer has a high resistance immediately after the growth, it is subsequently annealed in a vacuum or in an inert or inactive gas to change it into a p-type semiconductor layer. It is considered that the change into a p-type occurs because Mg in GaN is activated and releases carriers.
However, the carrier concentration of the p-type GaN layer obtained in the above-mentioned process is around only 3.times.10.sup.17 cm.sup.-3, and the resistance still remains undesirably high. Therefore, the use of a nitride III-V compound semiconductor in a semiconductor laser presents some difficulties. A first problem attendant on the use of a p-type GaN layer as a contact layer for the p-side electrode arises because the p-type GaN layer has a high resistance and a large voltage loss may occur in the p-type GaN layer when the laser is operated with a high electric current. A second problem arises because of a low carrier concentration of the p-type GaN layer, which causes a contact resistance as high as 10.sup.-2 cm.sup.2 between the p-type GaN layer and the p-side electrode. This causes a voltage loss of approximately 10 V along the interface between the p-type GaN layer and the p-side electrode while the semiconductor laser is operated in a typical inrush current density, 1 kA/cm.sup.2, and causes a deterioration in laser characteristics. A third problem is that the need for an annealing step for activating the impurity after growth of the Mg-doped GaN layer causes an increase in the number of steps required to perform the manufacturing process.
The above problems concerning p-type GaN also apply to fabrication of a p-type layer of any nitride III-V compound semiconductor or, more generally, a nitride compound semiconductor, other than GaN.